This invention relates to vitreous bonded cubic boron nitride abrasive articles and, in particular, to vitreous bonded cubic boron nitride grinding wheels.
Since the invention of cubic boron nitride it has been known in the art to use cubic boron nitride as an abrasive in grinding wheels. The hardness, strength and other properties of cubic boron nitride have made it a desirable, although expensive, abrasive for grinding wheel applications. Notwithstanding its expensive nature, cubic boron nitride has been found to be useful in the grinding of metals and other hard materials. Thus cubic nitride has been incorporated into metal bonded, organic resin bonded and vitreous bonded grinding wheels.
In the grinding of metals and other hard materials, such as metal carbides, it is highly important that the grinding wheel be strong, resist thermal shock, resist mechanical shock, exhibit low wear, hold shape, be free cutting; (i.e., resist being loaded up by the material being ground), have good grinding efficiency and exhibit good metal removal rate. All of these attributes are of particular importance in a grinding operation such as internal grinding of metal parts. It is, for example, particularly important in an internal grinding operation, such as the grinding of a bore in a metal part, that the grinding wheel maintain its shape for extended periods while exhibiting good strength, good grinding efficiency and good metal removal rate. Excessive and/or uneven wear of the grinding wheel causes out-of-tolerance dimensions and undesirable alteration of the shape of the workpiece being ground. Similar problems and requirements prevail in the contour grinding of metals with preshaped grinding wheels. As in internal grinding, contour grinding of metals with preshaped grinding wheels requires that the preshaped grinding wheel retain its initial configuration for long periods while having good grinding efficiency and metal removal rate. Of course, these desirable attributes of the grinding wheels apply to all types of grinding to varying degrees. In some types of grinding critical retention of grinding wheel shape is not as important as in other types of grinding, however, all grinding wheels desirably must have the attributes which render them efficient and economical in use.
Resin bonded cubic boron nitride grinding wheels have been found, commercially, to be unsatisfactory in strenuous, high precision grinding operations, such as internal grinding or contour grinding, because they readily lose their shape and have poor resistance to high temperatures often encountered under strenuous grinding conditions. Metal bonded cubic boron nitride grinding wheels are expensive and their use in the commercial grinding art has been confined principally to grinding very hard materials such as metal carbide cutting tool elements.
The high coefficient of expansion of the metal bond and the affinity toward loading have made the metal bonded cubic boron nitride grinding wheel undesirable for use in grinding metal for extended grinding periods under strenuous conditions, and exhibit poor cutting rates. Vitreous bonded cubic boron nitride grinding wheels have enjoyed only limited success in commercial metal grinding operations, especially wherein the grinding wheel is subjected to high mechanical and thermal shock, and required to maintain its shape over extended periods under strenuous grinding conditions. Bond failure, with subsequent loss of the cubic boron nitride abrasive grain, appear to be the principle cause for the poor performance of vitreous bonded cubic boron nitride grinding wheels under high thermal and mechanical shock grinding conditions. Additionally, it is known that the prior art vitreous bonded cubic boron nitride grinding wheels exhibit low grinding efficiency for many types of metals and often require relatively high grinding pressure or force to achieve grinding action.
It is desirable that in a vitreous bonded cubic boron nitride abrasive article, such as a grinding wheel, that it be free grinding (i.e., resist loading), grind at low grinding pressures, have good adhesion between the bond phase and the cubic born nitride so as to resist rapid or premature breaking out of the cubic boron nitride grain with subsequent rapid wear of the wheel and loss of its shape, be resistant to mechanical and thermal shock and have good grinding efficiency. The prior art vitreous bonded cubic boron nitride abrasive articles (e.g., grinding wheels) have been found wanting in one or more of the aforementioned properties.
It is an object of this invention to provide an improved vitreous bonded cubic boron nitride abrasive article.
It is another object of this invention to provide a vitreous bonded cubic boron nitride abrasive article which avoids many, if not all, of the disadvantages and undesirable features of prior art vitreous bonded cubic boron nitride abrasive articles.
It is further object of this invention to provide a vitreous cubic boron nitride grinding wheel with improved grinding efficiency, improved resistance to mechanical and thermal shock and improved retention of shape.
These and other objects of this invention will be made clear in the following description and examples of the invention.